Method and equipment for compressing a dispersion in liquid-liquid extraction

ABSTRACT

The invention relates to a method for controlling and compressing a dispersion formed in the mixing stage of liquid-liquid extraction. The compression of the dispersion is achieved by reducing the cross-section of the separation section in the direction of flow and by equipping the separation section with a dispersion-damming member. The equipment according to the invention thus comprises a settler, which is essentially trapezoid in shape so that its cross-section is wider at the feed end of the settler and gets smaller towards the rear end of the settler, from whence the separated solutions are discharged from the settler, and the settler is equipped with at least one device to dam up the dispersion. In particular the method and equipment relate to the extraction process used in the recovery of metals.

The invention relates to a method for controlling and compressing adispersion formed in the mixing stage of liquid-liquid extraction. Thecompression of the dispersion is achieved by reducing the cross-sectionof the separation section in the direction of flow and by equipping theseparation section with a dispersion-damming member. The equipmentaccording to the invention thus comprises a settler, which isessentially trapezoid in shape so that its cross-section is wider at thefeed end of the settler and gets smaller towards the rear end of thesettler, from whence the separated solutions are discharged from thesettler, and the settler is equipped with at least one device to dam upthe dispersion. In particular the method and equipment relate to theextraction process used in the recovery of metals.

Liquid-liquid extraction has been used earlier in the metallurgicalindustry typically in the processing of solutions with a weak valuablemetal content. Many large copper and uranium recovery extraction plantsfall into this category. With regard to copper, however, the situationis changing, because the extraction feed solutions are becomingnoticeably stronger with the pressurised concentrate leaching processescoming into operation. Likewise some cobalt and zinc extractionprocesses also treat strong feed solutions. Nevertheless, the size ofthe equipment, particularly in the case of copper, will generally remainlarge, also in the new pressurised leaching processes.

In all extraction processes a valuable metal-containing aqueous solutionis brought into contact with an organic solution in the extractionmixing section, forming a dispersion of two solutions that are insolublein each other. The solutions in the dispersion are separated from eachother in the separation section of extraction, where the solutionsseparate from each other into two layers with a dispersion bandremaining between them. During the mixing stage, either one or more ofthe valuable metals in the aqueous solution is transferred to theorganic phase, from which the valuable metals are recovered by strippingthe aqueous solution. Extraction is performed in an equipment, where themixing and separation sections are either located one on top of theother (column) or in series on more or less the same level horizontally.Almost always in cases when large-scale extractions of weak solutionsare concerned, such as copper extraction, the equipment is positioned inan essentially horizontal position. When we refer to extractionhereinafter, the term is used for various arrangements, but essentiallyequipment in the same level.

The separation section in extraction is conventionally rectangular incross-section or in some special cases square. For instance in U.S. Pat.No. 6,132,615 the settler described is of rectangular shape. The settleris equipped with several picket fences.

Now a method has been developed for controlling and compressing adispersion formed in the mixing stage of liquid-liquid extraction. Thecompression of the dispersion is achieved by reducing the cross-sectionof the separation section in the direction of flow and by equipping theseparation section with a dispersion-damming member. The equipmentaccording to the invention thus comprises a settler, which is basicallytrapezoid in shape so that its cross-section is wider at the feed end ofthe settler and gets smaller towards the rear end of the settler, fromwhence the separated solutions are discharged from the settler, and thesettler is equipped with at least one device to dam up the dispersion.In particular the method and equipment relate to the extraction processused in the recovery of metals.

The essential features of the invention will be made apparent in theattached claims.

In the mixing section of extraction equipment i.e. in the mixer, adispersion formed of two extraction liquids is fed into the separationsection i.e. settler. It is clear that the portion of the dispersion inthe front end of the settler is dominant. However, it is required tocompress the dispersion so the cross-section of the settler is reducedin the direction of flow and at the same time the settler is equippedwith at least one member which will in addition accomplish the dammingand compressing of the dispersion. The member causing the damming islocated in the settler, essentially in the direction of the end walls,the feed end and rear end. The damming member can be either atraditional picket fence or preferably a revert member, with the effectof turning the flow direction of the dispersion to a mainly verticalone. The damming member compresses the dispersion in the direction offlow and the elevation of the dispersion, and the settler with itsdiminishing cross-section in the direction of flow achieves furthercompression laterally.

The reduction of the cross-sectional area of flow in the separationsection is at the same time a method to even out the dispersion streamand to control the progression of the dispersion towards the rear end ofthe separation section in the intermediate spaces between the dammingmembers. As the stream moves forward in the separation section and thesolutions separate from the dispersion into their own layers, thedispersion band becomes thinner unless the cross-sectional area isreduced. Gravity for its part evens out the thickness of the dispersionlayer, because it causes the dispersion to flow towards the rear end ofthe separation section. However, this correspondingly weakens theseparation capacity of the settler, especially as all additional flowmovements in themselves slow down separation.

Using the method and equipment of this invention, solutions separatingfrom each other can be made to move forward at an even rate in both alateral and vertical direction in conditions arranged for separation.These include the fact that the solutions are made to move forward in aplug flow manner from the front end of the separation section to thetail end. One aim of the method in the present invention is to speed upthe separation of the solutions from the dispersion and to improve thefinal separation efficiency of the solutions i.e. to reduce theentrainment of each solution in the other. A compact dispersion formedby damming enables an improved degree of solution separation i.e. theamount of entrainment in each solution is decreased. The flow movementachieved by gravity also lessens the impact of a compact dispersion.

A thick dispersion band at the front end of the separation sectionpromotes good droplet separation. Thus it is preferable that thethickness of the dispersion band at least in the first third of theseparation section is about 75% of the height of the solution anddecreases gradually.

The reduction of the cross-section of the flow in the separation sectionin the direction of flow causes the flow rate of the solution toincrease. The rise in flow rate of the aqueous solution towards the rearend of the separation section keeps the aqueous solution flowingforwards evenly along the bottom layer. The linear speed of the organicsolution is not of course allowed to increase above the critical limit,above which the amount of entrainment of aqueous solution in the organicsolution begins to grow. The linear speed limit of the organic solutionin this case can be considered to be 70 mm/s.

The shape of the settler, where the cross-section of the feed end iswider than the rear end, can be achieved in many ways. One such is atrapezium that narrows symmetrically on both sides. The cross-section ofthe settler can also be reduced asymmetrically, for example a trapeziumcan also narrow on only one side, whereby one side of the settler isperpendicular to both the feed end and the rear end, but the other sideis in an oblique position in relation to the end walls. The reduction ofthe cross-section is in proportion to the depth of the settler also, sothat at its smallest, the width of the rear end of the settler is in therange of 30-60% of the width of the feed end.

The settler, with a cross-section decreasing in the direction of flow,is of course equipped with the appropriate equipment. Thus one orseveral picket fences or other equivalent elements are located in thesettler feed end, enabling a dispersion fed mainly to one point to bedistributed evenly over the whole of the cross-section of the settler.

In addition, so that the dispersion fed into the separation section isspread over the whole of the cross-section of the settler it is alsoequipped with at least one member to dam up the dispersion. Thetraditional picket fence that can be used was mentioned earlier. Insteadof a picket fence, the separation section or settler can be equippedwith at least one revert member or both can be used. The preferrednumber of revert members is 3-5.

Using a revert member, the phases separated from the dispersion are madeto flow relatively freely along the longitudinal axis of the separationsection, but the unseparated dispersion is dammed up using a dammingrevert member placed in the separation section. The revert memberextends right up to the sidewalls of the separation section. Theequipment in accordance with the invention comprises at least one revertmember located in the settler (separation section), said revert membercomprising at least two, plate-like sections, or revert plates, whichare at different heights and essentially set perpendicularly to thelongitudinal axis of the settler (in the direction of the solutionflow). In the area formed between the revert plates, the revert channel,the direction of the dispersion flow is almost vertical, because thedispersion is made to flow above or below each revert plate into therevert channel. Changing the direction of flow of the dispersion atleast once in the separation section improves the separation of thedispersion into pure solution layers above and below the dispersion. Therevert member can be positioned at different stages of extraction, suchas both in the actual extraction and also in any washing and strippingseparation sections.

The dispersion stream is prevented from flowing forwards directly in thesettler by arranging at least one revert member extending above theseparation section. In order for the dispersion to move past the revertmember, in the first stage it must be pressed against the firstplate-like part of the revert member and under it into the revertchannel, which is formed between the plate-like parts of the revertmember. From the revert channel the dispersion surface is made to riseso that it is able to flow over the second plate-like part of the revertmember. The number of revert members in the separation part of theextraction is at least one, but can vary for instance between 1 and 6.There are at least two plate-like parts in one revert member, but thenumber of said parts can also vary, for instance between 2 and 6. Thefirst plate-like part of the revert member and subsequently every secondpart is located essentially higher in the separation section than thesecond plate-like part and every other part after that.

The first plate-like part belonging to the revert member, i.e. the firstrevert plate, is located in the separation section at a level where itsupper edge extends above the dispersion band into the organic solutionphase. When the separated solutions and the dispersion band between themflow from the feed end of the separation section towards the dischargeend, the dispersion band is pressed against the first revert plate. Thepositioning of the revert plates determines the desired thickness of theorganic solution layer. The dispersion should accumulate in suchquantities that because it is heavier than the separated organicsolution it penetrates through the riser channel or channels between therevert plates to the next section of the separation section, where thethickness of the layer of separated solutions is greater than in theprevious section. The organic and aqueous solutions that have alreadyseparated into their own phases, are able to flow freely at the revertmember into the next section of the separation section, but thedispersion has to collect in a layer of sufficient thickness before itis able to enter the next section of the separation section via therevert member. The dispersion moves forward only when the separationsection is charged with a sufficiently large flow. The larger thesettler, the larger the flow required.

The first revert plate is mainly solid, but has vertical slots in itsupper section, which ensure an even flow-through of the organic solutionat the revert member along the whole length of the separation section.The first revert plate extends above the surface of the organicsolution, as do the slots in its upper edge. The slots going down fromthe top edge of the revert plate reach a depth equivalent to a maximumof half that of the thickness of the layer of separated organicsolution. The slotted zone accounts for about 5-15% of the total heightof the revert plate. The lower edge of the first revert plate extends tothe bottom part of the separation section, but however to such adistance from the bottom that it is within the prevailing dispersionlayer. The distance of the lower edge from the bottom is greater thefurther away the revert member is from the feed end of the separationsection. In practice, the lower edge of the first revert plate is at adistance from the bottom that is 12-50% of the total depth of solutionin the separation section (settler).

The second revert plate of the revert member is the same type as thefirst i.e. basically solid. The lower edge of the second revert plate isplaced far lower than the lower edge of the first revert plate, buthowever, so that there is space for the separated aqueous solution toflow unimpeded. The distance of the lower edge of the second revertplate from the bottom depends on the location of the revert member inthe separation section. The lower edge of the revert plate is higher inthe separation section, the further away the revert member is from thefeed end of the separation section. In practice, the lower edge of thesecond revert plate is at a distance from the bottom that is 5-35% ofthe depth of solution in the separation section. The upper edge of thesecond revert plate is located below the surface of the organicsolution, and the distance of the upper edge from the surface of theorganic solution is greater the further away the revert member is fromthe feed end of the separation section. In practice, the upper edge ofthe second revert plate is at a distance below the surface of thesolution that is 12-35% of the total solution depth in the separationsection.

The even distribution of the dispersion into the revert channel and aneven flow out of it is made easier if the lower end of the first revertplate of the revert member is also provided with an equivalent type ofslotted zone as that on the upper end of the same revert plate.Likewise, it is preferable to furnish the top end of the second revertplate with a slotted zone and the purpose of the slots in this case toois to promote the even distribution of the dispersion into theseparation section. If the revert member is composed of several revertplates, the slotted zones are located on the upper and lower edges ofthe corresponding plates. The height of the slotted zones on the loweredge of the first revert plate and the upper edge of the second plate isin the range of 5-15% of the height of the revert plate.

If the revert member is made up of more than two revert plates, thebottom clearance of the lower edge of the third revert plate is 0-30%larger than that of the first revert plate. The distance of the thirdrevert plate from the surface of the organic solution is 10-30% smallerthan the distance of the second plate. Both the bottom clearance and thedistance from the surface of the organic solution of the fourth revertplate are 0-30% greater than that of the second revert plate.

The use of a revert member reduces the amount of organic solutionentrainment in the aqueous solution, so that the entrainment content inthe aqueous solution entering stripping remains less than 10 ppm,generally between 2-7 ppm. For example, in copper extraction therecovery of copper takes place by electrolysis in an electrowinningcircuit. The electrolysis process cannot tolerate an organic solution,and if the solution entering electrolysis is not sufficiently pure, itmust be purified for example by flotation or pressure filtration. Aseparation section with a diminishing cross-section and the use of arevert member facilitates in particular the direct routing of thesolution produced in extraction to further processing without separatepurification stages.

An arrangement in accordance with this invention enables the reductionof the amount of the dispersion that remains unseparated at the tail endof the separation section, so that it is at most 10% of the thickness ofthe solutions in the separation section. It is also possible using thismethod to regulate the thickness of the layer of organic solution. Thethickness of the layer of organic solution is regulated gradually inaccordance with the number of revert members used.

The method and equipment are intended particularly for the extraction ofmetals, where the metal to be recovered is one of the following: copper,uranium, cobalt, nickel or zinc.

The invention is described further in the attached drawings, where

FIG. 1 is a cross-section of an embodiment of a settler in accordancewith the invention,

FIG. 2 is a cross-section of another embodiment of a settler inaccordance with the invention,

FIG. 3 is a vertical section of a settler, equipped with revert members,and

FIGS. 4-6 show a cross-section of a settler, where a revert member islocated.

FIGS. 1 and 2 show a settler 1, which is composed essentially of avertical feed end 2, rear end 3, sidewalls 4 and 5. In FIG. 1 both thesidewalls 4 and 5 narrow in the direction of the flow in the settler. InFIG. 2 one sidewall is perpendicular to the feed and rear ends, and theother is placed at an angle to them so that the cross-sectional area ofthe settler diminishes in the direction of flow. The dispersion feedconnection 6 is also shown in the drawing, which is connected at one endto the extraction mixing section (not shown in the drawing). In the feedend 2 of the settler there is a picket fence or other suitabledirectional element 7, with which the dispersion is spread across thewhole width of the settler. The settler is also equipped with onedamming member 8. The separated solutions are discharged at the rear end3 of the settler, where first in the direction of flow there is theorganic solution launder 9, into which the organic stream flows as theoverflow and is routed onwards from there. The aqueous solution iscollected in what is known as a water end 10, to which the aqueoussolution flows below the organic solution launder.

FIG. 3 shows a side elevation of the settler in FIGS. 1 and 2. Thedrawing shows that in the front end of the settler the solutions haveseparated into their own phases only a little, the organic solution 11above the dispersion 12 and the aqueous solution 13 under the dispersionon the bottom 14 of the settler. The dispersion band is dominant at thefront end of the settler. In this case damming revert members 8 arefurther located in the settler, in this case three of them. Each revertmember is composed of two revert plates 15 and 16 and the revert channel17 formed between them. The revert plates are located in the settler sothat they lie crosswise in relation to the longitudinal axis (thedirection of flow), i.e. in the same direction as the feed and rear endof the settler. The section of the revert plates with vertical slots isshown in the side elevation by a dotted line; the rest of the plates aresolid. Thus the top 17 of the first revert plate 15 is provided withvertical slots as is the bottom 18 of the plate and the top 19 of thesecond revert plate. The position of the revert member in the settlercan be determined as required.

FIG. 3 shows that the distance of the revert plates from each other canalso be changed so that the distance between them becomes smaller in thedirection of flow. The distance between the plates is measured in such away that the flow rate of the dispersion in the revert channel betweenthem is of the order of 0.05-0.4 m/s. The vertical slots of the top ofthe first revert plate are dimensioned equally over the cross-section ofthe settler so that the flow rate of the organic phase flowing throughthem is in the range of 0.1-0.6 m/s.

FIG. 4 is an example of one revert member 21 placed in a settler 20,which comprises two revert plates 22 and 23. The first revert plate 22extends above the surface 24 of the organic solution. To simplify thedrawing the vertical slotted section is not shown separately. Seen inthe direction of flow, baffle plates 25 and 26 are located behind theupper part of each revert plate, made up of vertical plate strips, whichare placed in between the gaps in the top of the revert plate. Verticalflow channels are formed between the strips, with a width much greaterthan that of the strips themselves. The baffle plates are in the form ofa downward-facing comb, so that their upper edge is fixed. The distanceof the baffle plates from the revert plate is 2-3 times the width of therevert plate slot. In this way it is possible to slow down and even outthe flow entering the settler extension, which is conducive forimproving the separating properties of the settler. The height of thebaffle plates can be changed.

FIG. 5 represents a simplified settler 27, into which one revert member28 has been placed, which in this case consists of four revert plates29, 30, 31 and 32. Baffle plates 33 and 34 have again been placed infront of the top of the first and last revert plates. In the case ofthis drawing the dispersion flow has to flow through three revertchannels, 35, 36 and 37, wherein the flow is almost vertical, eitherupward or downward. Vertical flows are excellent for making solutionsseparate from each other. During vertical flow small droplets inparticular are brought into contact with their own phase and combinewith it.

FIG. 6 shows an example of a settler 38 with only one revert member 39,which is formed of two revert plates 40 and 41. The revert plates arenow positioned at an angle rather than being upright, but neverthelessthe dispersion band moving forward in revert channel 42 in effect has torise vertically between the revert plates. Revert plates can thereforebe positioned at an angle of between 50° and 90° to the horizontal. Theinclination may be either towards the settler feed end as in FIG. 6 orthey may be inclined towards the rear end of the settler. Theinclination shown in FIG. 6 is a better alternative than the latter. Itis appropriate to use inclined revert members when treating solutionsthat separate poorly.

1. A method for controlling and compressing a dispersion formed from anaqueous solution and an organic solution in connection with the recoveryof metals in a liquid-liquid extraction separation section, comprisingreducing the cross-sectional area of the separation section in thedirection of flow and damming up the dispersion using at least onedamming member located in the separation section, to compress thedispersion.
 2. The method according to claim 1, further comprisingreducing symmetrically the cross-section of the separation section. 3.The method according to claim 1, further comprising reducingasymmetrically the cross-section of the separation section.
 4. Themethod according to claim 1, wherein the metal to be recovered isselected from the group consisting of copper, uranium, cobalt, nickeland zinc.
 5. The method according to claim 1, further comprising causingthe dispersion to dam up using at least one revert member extending fromone side to the other placed in the separation section, said revertmember being made up of at least two plate-like parts, and causing thedirection of the dispersion to turn essentially vertically in the revertchannel between them.
 6. The method according to claim 5, furthercomprising extending the upper edge of the first plate-like part of therevert member into the organic solution and causing the organic solutionto flow partially through the slotted zone arranged in the top of theplate-like part.
 7. The method according to claim 5, further comprisingcausing the dispersion stream to dam up using the first plate-like partof the revert member and to flow under the first plate-like part intothe revert channel.
 8. The method according to claim 5, furthercomprising causing wherein the dispersion that has flowed into therevert member to flow over the last plate-like part of the revert memberinto the separation section after the revert member.
 9. The methodaccording to claim 5, wherein the number of revert members located inthe separation section is 1-6.
 10. The method according to claim 5,wherein the number of plate-like parts in the revert member is 2-6. 11.Equipment for controlling and compressing a dispersion formed from anaqueous solution and an organic solution in connection with the recoveryof metals in a liquid-liquid extraction process settler, which comprisesa feed end, sidewalls, a rear end and a bottom, the cross-section of thesettler reducing in the direction of flow and the settler being equippedwith at least one damming member.
 12. The equipment according to claim11, wherein the cross-section of the settler is a trapezium.
 13. Theequipment according to claim 11, wherein the settler is equipped with apicket fence.
 14. The equipment according to claim 11, wherein thesettler is equipped with at least one revert member, positioned from onesidewall to the other crosswise in relation to the longitudinal axis ofthe settler, said revert member comprising at least two revert plateslocated at different heights.
 15. The equipment according to claim 11,wherein the number of revert members is 1-6.
 16. The equipment accordingto claim 11, wherein the number of revert plates in the revert member is2-6.
 17. The equipment according to claim 11, wherein the first revertplate of the revert member and subsequently every second revert plate islocated higher than the second one and every other plate after that. 18.The equipment according to claim 11, wherein the upper edge of the firstrevert plate is located above the surface of the liquid in the settler.19. The equipment according to claim 11, wherein the distance of thelower edge of the first revert plate from the bottom of the settler is12-50% of the depth of the solution in the settler.
 20. The equipmentaccording to claim 11, wherein the revert plates are mainly solid whenseen in elevation.
 21. The equipment according to claim 11, wherein theupper edge of the first revert plate consists of vertical slots of alength which corresponds to 5-15% of the height of the revert plateconcerned.
 22. The equipment according to claim 11, wherein the loweredge of the first and thereafter every second revert plate consists ofvertical slots of a length which corresponds to 5-15% of the height ofthe revert plate concerned.
 23. The equipment according to claim 11,wherein the upper edge of the second and subsequently every other revertplate consists of vertical slots of a length which corresponds to 5-15%of the height of the revert plate concerned.
 24. The equipment accordingto claim 11, wherein the distance of the lower edge of the second revertplate from the bottom of the settler is 5-35% of the depth of thesolution in the settler.
 25. The equipment according to claim 11,wherein the upper edge of the second revert plate is located below thesurface of the solution, at a distance which is 12-35% of the depth ofthe solution in the settler.
 26. The equipment according to claim 11,wherein the revert plates of the revert member are located in thesettler at an angle of 50-90° in relation to the horizontal.
 27. Theequipment according to claim 11, wherein the feed end of the settler isequipped with a picket fence.